Split anode plasma accelerometer



Nov. 12, 1963 s. e. HUGHES 3,110,185

SPLIT ANODE PLASMA ACCELEROMETER Filed Oct. 10, 1960 FlG.l

FIGB P. 7 FIG.2 6 I 5 e Z M POTENTIAL 5 SOURCE n. 5 O 5 E E .05 .I .I5 .2 ACCELERATING FORCE m s's ti /4 3o I up! 1-1 SOURCE L INVENTOR! ,-4 STANLEY s. HUGHES,

: BY WW Y AXIS HIS ATTORNEY.

1 3a ,39 I POTENTIAL ZS United States Patent Ofice 3,1 lfiyi5 Patented Nov. 12, 1953 3,116,185 SPLIT ANGDE PLAflMA ACCELERGMETER Stanley G. Hughes, Devon, Pa, assignor to General Electrio Company, a corporation of New York Filed Get. 18, 1960, Ser- No. 61,451 7 Claims. (i. 73-517) The present invention relates to a novel device that is responsive to acceleration forces and more particularly to a novel plasma electron device which may be employed in an electrical circuit as an accelerometer, velocimeter or inclinometer.

The present invention constitutes an improvement of an acceleration responsive device of the type disclosed in applicants copending application entitled Plasma Accelerometer, cerial No. 61,465, filed October 10, 1960, assigned to the assignee of the present invention. In said copending application there is provided a plasma electron device wherein the ion flow between electrodes is responsive to applied acceleration forces in a predetermined direction. Mechanical means are employed to stabilize said ion flow and provide an output response linearly related to the applied acceleration forces. The present invention obviates the necessity for such mechanical means and obtains an improved device.

Accordingly, one object of the invention is to provide a novel acceleration responsive device having essentially no moving mechanical parts, which is reliable, rugged and inexpensive to manufacture.

It is another object of the invention to provide a novel plasma electron device having essentially no moving mechanical parts and in which the ion flow between electrodes functions as a seismic mass, and which may be employed as a relatively frictionless and rugged accelerometer, velocimeter or inclinometer.

It is a further object of the invention to provide a novel plasma electron device in which the ion flow between electrodes functions as the seismic mass and which may be employed to measure the right angle components of acceleration forces in any direction in a single plane.

Briefly, in accordance with one aspect of the invention, there is provided an electrical circuit employing an electron device responsive to acceleration forces in a predetermined direction which comprises a hermetically sealed envelope filled with a plasma or fluid and containing a cathode electrode having a restricted emissive area and a pair of closely spaced flat anode plates. A portion of said cathode projects through a central aperture formed by said closely spaced anode plates. A source of energizing potential is coupled by first and second load resistors respectively to said anode plates for producing, in the absence of said acceleration forces, a flow of electrical charges or ions from the emissive tip of the projecting portion of said cathode equally to each anode. Said ion flow is found to shift in response to said acceleration forces, causing unequal conduction to said anode plates. One explanation for this phenomenon of the ion path shifting in response to the application of acceleration forces is that said forces cause a density gradient to exist in the enclosed fluid which in combination with the heating of the fluid, primarily at the emissive area of the cathode, produces a component of convection flow in the fluid. The convection flow assumes a closed path away from the source of heat towards the region of low density and return through the unheated and high density region. Convection flow in this manner acts as a force upon said ions to shift their path towards the less dense region. This effect per se is well known as the operative principle underlying the familiar horn gap arrester. It is, of course, clear that it is essential that a change in density of the fluid with increase in temperature be accompanied by an increase in its electrical conductivity for this phenomenon to occur. This is necessary so that the electrical flow will tend to be concentrated in the paths of warmer fluid and will therefore tend to keep those paths warmer than the surrounding fluid. If the conductivity of the fluid were everywhere perfectly uniform, the electric current flow would, obviously, be unaffected by mechanical convection currents in the fluid; merely being thermally convective is not suflicient to assure the effect.

The unequal ion flow, or conduction, develops unequal voltages at each of said anodes which sets up an electric field to oppose said unequal conduction. Thus, the ion flow is stabilized so that it is linearly related to the applied acceleration forces. A voltmeter connected across the anode plates is responsive to the unequal conduction and provides a measure of said applied acceleration forces.

In accordance with a further aspect of the invention four quartered anode plates are employed in lieu of the previous two plates so that voltages developed between two pairs of said anode plates, resulting from unequal conduction to said plates, provide a measure of acceleration component along two axes disposed at right angles to one another. Thus, the two right angle components of resultant acceleration forces in any direction in a single plane may be resolved.

The invention will be better understood from the following description taken in connection with the accompanying drawings while the novel and distinct features of the invention are particularly pointed out in the appended claims.

FIGURE 1 is a schematic diagram of one embodiment of applicants invention which may be employed to measure applied acceleration forces;

FIGURE 2 is a perspective view of the anode structure of the novel electron device shown in FIGURE 1;

FIGURE 3 is a graph illustrating the output response experimentally obtained for the circuit of FIGURE 1;

FIGURE 4 is a perspective view of the novel electron device of FIGURE 1;

FIGURE 5 is a cross-sectional view of the electrode assembly of the device of FIGURE 4 taken along the lines 5-5;

FIGURE 6 is a schematic diagram of a second embodiment of applicants invention which may be employed to measure applied acceleration forces in any direction in a single plane or tilt around any axis in a single plane; and

FIGURE 7 is a perspective view of the anode structure of the novel electron device of FIGURE 6.

Referring now to FIGURE 1, there is illustrated in schematic form a circuit diagram of appllcants accelerometer, or inclinometer, which comprises an electron device 1 wherein the direction of ion flow is responsive to acceleration forces applied to the discharge device in either of two directions, e.g., indicated by the arrows a and b parallel to the X axis, and, accordingly, when placed in a gravitational field, the device is also responsive to the inclinations around the Y axis, perpendicular to the plane of the paper, e.g., indicated by the arrows c and d. Thus, the responsive direction in FIGURE 1 may be thought of as parallel to the X axis for applied acceleration forces, and as around the Y axis for applied tilts. The electron device 'lcomprises an enclosed envelope 2 filled with a plasma medium 3, which may take the form of an inert gaseous material or a conducting liquid, and having an electron emissive cathode 4- and a split disc anode composed of portions 5 and 6, shown in a perspective view in FIGURE 2. The cathode 4 is symmetrically mounted with respect to anode portions 5 and 6 so that in the absence of an accelerating force, or an equivalent tilt, in the responsive direction, the flow of electrons from the cathode 4 to each of the anode portions 5 and 6 is equal. A detailed showing of one operable form of the electron device 1 will be discussed with reference to FIGURES 4 and 5.

The anode portions 5 and 6 are energized from a source of potential 7 through load resistors 8 and 9, respectively. A direct voltage source is preferred, although a low frequency source, e.g., 60 cycles has been employed successfully. Resistor S is connected between the anode portion 5 and the positive terminal of source 7, and the resistor 9 is connected between the anode portion 6 and said positive terminal. The negative terminal of the source 7 is connected through current limiting resistor 10 to the cathode 4 to complete the circuit. Voltages are developed across load resistors 8 and 9 in accordance with the conduction from the cathode 4 to the anode portions 5 and 6 so that with no acceleration forces applied, the equally conducting anodes develop equal voltages. An applied accelerating force is found to'produce unequal conduction which results in unequal voltages across resistors 8 and 9. A voltmeter 11 is connected across the anode portions 5 and 6 for measuring the voltage difference between these electrodes, thus providing an indication of the acceleration forces applied to the electron device 1, as will be further explained in greater detail.

Considering the operation of the circuit of FIGURE 1 when employing one form of applicants gaseous discharge device, in the absence of any component of acceleration forces in the responsive direction, the electrons emitted from the cathode 4 will produce a concentrated glow dis charge, ionizing the gas 3 along a relatively narrow path from the upper tip of the cathode 4 to a concentrated area centered upon one of the two boundries between the anode portions 5 and 6 along the path 32, as shown in FIGURE 4. It is noted that upon initial energization of the device, should the discharge path tend to assume a different orientation, it will instantly respond to the electric field forces applied by the anode portions and lock into the prescribed position, for reasons to be subsequently considered. In the prescribed position, current flow between the cathode 4 and the anode portions 5 and 6 will be equal, which provides equal voltage drops'across resistors 25 and 9. This produces a null condition in voltmeter 11, indicative of zero applied acceleration forces in the directions of the arrows a and has well as an indication of a zero applied tilt. Now considering an accelerating force applied to the electron device 1 in the direction shown by the arrow at,

the orientation of the discharge path from the tip of the cathode 4 to the anode portions 5 and 6 will be found to shift from the previous equal conduction path, towards the anode portion 5. Thus, conduction tends to increase at anode portion 5 and to decrease at anode portion 6. A voltage differential occurs between anode portions 5 and 6 which is measured by the voltmeter 11;

It is theorized that this differential conduction phenomenon is a result of thermal effects and convection flow of the molecules in the gaseous material 3. Convection flow is caused by the combined effect of the heating phenomenon occurring at the electron emitting tip of the cathode 4 and a density gradient in the gas 3 produced by the applied accelerating forces. The direction of convection flow is determined by the resultant accelerating forces acting on the device. An applied force produces a density gradient in the direction of the force causing convection flow in closed paths away from the heated region towards the region of low density and return through the cooler and high density region. The convection fiow is assumed to, first influence the position of the cathode sheath, an accumulation of positive ions normally formed in proximity to the cathode which determines the initial direction of the emitted electrons, and second, to influence the direction of the ions in the discharge path. This influence is due to the efiect of the physical movement of the gas molecules on the ions of the cathode sheath and the discharge path. Thus, with no accelerating forces present, there s no convection flow and the discharge path is determined simply by the electric field'present. Since the cathode is positioned symmetrically with respect to the anode portions, the electric field provides a discharge path of equal conduction to each of the anode portions. With only a gravitation force present and assuming the electron device to be vertically oriented, convection flow may be seen to be in a direction upward and away from the heated regions 7 and is symmetrical with respect to the anode portions.

There is no component of flow across the discharge path and hence no shift of the path. With an accelerating force applied in the direction of e arrow at, there is produced a transverse component of convection How, in the same direction as the accelerating force, which shifts the discharge path towards the anode portion 5, producing the aforementioned voltage difierential,

This dilference voltage provides a counterbalancing, or negative feedback effect, which acts to oppose the differential current flow to the anode portions 5 and 6 caused by the transverse component of the convection flow and other effects. This feedback effect maintains a linear relationship between the difierence voltage, which is the output of the circuit, and the applied acceleration forces. It also serves to make the output less dependent on parameters such as the discharge current'and the gas pressure. Thus, as conduction to the anode portion 5 tends to increase because of the convection flow, the anode voltage, providing the electric field between the cathode 4 and anode 5, decreases. In a comparable manner, the decreased conduction to anode portion 6 eflects a slightly greater voltage at this anode, which increases the electric field between the cathode andanode 6. The difierence in electric fields tends to attract the ions in the direction of the anode portion 6 and counter-balances the previous tendency of increased conduction to anode portion 5 by convection. Thus, the discharge path between the cathode and the anode portions becomes stabilized at a position determined by the applied accelerating force.

The measured voltage of voltmeter 11 has been found to be linearly related within one percent to applied accelerating forces up to .2 G5, as indicated in FIGURE 3. In addition, fifty volt outputs have been obtained for inputs of 1 G so that applicants device has been found to be sensitive to minute acceleration forces in the range of millionths of a G. The sensitivity of the circuit may be adjusted by controlling the potential source, the value of resistors 8 to it) and the plasma medium. Further, the output has been shown to remain relatively constant within .005 Gs (in response to a constant input applied for a period of two hours. 7

It may be appreciated that if the electron device'l is inclined in a clockwise direction around the axis Y, shown by the arrow 0. in FIGURE 1, a differential conduction to the anode portions 5 and 6 will take place in a similar manner as with the accelerating force applied in the above explanation. Gravitational forces provide a convection flow towards anode portion 5 which biases conduction towards this anode portion, the conduction being stabilized by the feedback voltage generated;

With an applied accelerating force in the direction in-' dioated by the arrow 17, or with an inclination around axis Y in the direction of arrows c, a converse effect relative to that previously described is obtained. For this condition, conduction to the anode portion 6 will tend to predominate as a result of the convection flow, the conduction being stabilized by the greater anode voltage at anode portion 5.

The circuit of FIGURE 1 is relatively insensitive to force components applied in directions other than the directions of the arrows a and b or to inclinations other than around axis Y. Thus, cross modulation is minimized. For example, considering a force parallel to the plane of the anode portions but in the direction of axis Y, any resulting movement of the discharge path will be symmetrical with respect to the anode portions 5 and 6. Conduction to these portions will not change and no output voltage will be recorded by voltmeter ll. Similarly, for accelerating forces in a direction perpendicular to the plane of the anode no differential conduction will result. Thus, it may be seen that the device is sensitive only to forces parallel to the plane of the anode portions and orthogonal to the boundary lines between these portions.

The following parameter values are typical of one form of operative embodiment of applicants circuit when employing a gas composition of 95% argon and 5% nitrogen at a pressure of approximately 150 to 200 millimeters of mercury. They are given for purposes of illustration and are not to be construed as limiting.

Potential source '7 volts 1009 Resistors 8 and 9 ohms 100,000 Resistor it} do 306,000 Discharge current milliamperes 2 In FIGURES 4 and 5 is shown in detail an exemplary form of electron device that may be employed in the circuit of FIGURE 1. The device 1 comprises a base 2% which supports the electrode assembly, shown in cross section in FIGURE 5, and also serves to introduce the energizing leads to the chamber enclosed by envelope 2. The base 2% has a central circular platform 21 bordered by an annular groove 22. It is preferably composed of glass or other insulating material, such as a ceramic. The platform 21 has a central circular hollow in which is firmly mounted the lower portion of the electrode assembly. Through the platform extend the energizing leads 25 and 26 which are connected to anode portions 5 and 6, respectively. The lead 24 connecting to the cathode 4 is introduced through the circular hollow. The energizing leads are each hermetically sealed by the glass base. The envelope 2 may be glass, or a ceramic material of the same type as the base, and is hermetically sealed in the groove 22 along its bottom edge so as to form an airtight chamber for the electrode assembly.

The envelope 2. is filled with a plasma 3, which may be a gaseous or liquid medium. in the operative embodiments being considered, the medium 3 is a gas composed of 95 argon and 5% nitrogen under a pressure in the range of 100 to 250 millimeters of mercury. The pressure employed determines the form of discharge obtained, as will be presently discussed. The recited gas composition has been found to give satisfactory results, providing good output sensitivity and a minimum of undesirable gaseous deposits on the electrode structure. However, it is anticipated that other gas compositions may readily become known to skilled artisans which give equal and perhaps hnproved results.

The electrode assembly, shown in FIGURE 5 in a cross sectional view taken along the plme 55 in Fl URE 4, comprises an elongated, rod shaped cold cathode 5-, which is constructed of an electron emissive material such as tungsten. The cathode is surrounded by an insulating cenamic material 27 which restricts the electron emissive area of the cathode surface to the upper tip 28. The insulating material 27 also serves to support the anode portions 5 and 6 which are closely spaced semicircular fiat plates, as shown in FEGURE 2. These plates are of a rhodium plated nickel material having a smooth top surface. They are shown secured to the supporting insulating member 27 by terminal screws 29 and 36, which also provide electrical connection to anodes 5 land 6 by being connected to conductors '25 and 26. The insulating material has upper shoulders which fit over a pant of the top surface of the anode portions so that the ion flow is to a smooth anode surface. In the upper region of the insulating material 27 is concentrically embedded a second insulator 31, preferably an aluminum oxide material which is tough and therefore resistant to wear by electron and ion bombardment. The material 31 surrounds the uppermost part of cathode 4 and has a conical depression cut therein through which the projecting tip 23 of the cathode extends.

it has been found that by using the electrode structure illustrated, a discharge path is created between a restricted area of the cathode tip 28 and the anode portions illustnated by path 32 in FIGURE 4, without unwanted deposits being formed on the insulating material surrounding the cathode tip 23 by the electrons emitted therefrom. At the present time it seems necessary that the emissive portion of the cathode structure be limited to a restricted area so that the discharge path may be readily slufted in response to the applied acceleration forces. Thus, if too large an area is emissive, a sticking phenomenon of the discharge is noticed which causes erratic and nonlinear shifting of the discharge.

Although the illustrated form of electrode assembly has been found to yield satisfactory results, the invention should not be construed as limited to this precise structure since other structural forms may provide equally satisfactory operation. For example, the cathode tip may alternatively be positioned below the anode structure, the discharge path extending up to the lower surface of the anode portions. Further, the anode electrodes may take the form of split rings. These latter arrangements would require a somewhat modified mounting structure for the electrodes.

he type of discharge that occurs is a property of the pressure of the gas Within the envelope 2. It has been found that at a pressure of 150 to 250 millimeters of mercury there is formed a concentrated glow discharge of the type we have been primarily considering. Between pressures of to millimeters a ring type of glow discharge is obtained. The conduction from the cathode to the anode portions follows an umbrella like path so as to deposit the electrons in a ring on the surface of the anode portion. For this form of discharge, the circuit operation is basically the same as with the concentrated discharge. However, the ring path remains in essentially the same position, and the intensity of the discharge, being uniform in the absences of acceleration forces, tends to shift between the anode portions in response to said forces. At pressures below 100 millimeters of mercury the sensitivity to accelerating forces has been found to decrease appreciably.

In FIGURE 6 is illustrated a schematic diagram of a second embodiment of applicants plasma accelerometer sensitive to acceleration forces in two orthogonal directions, which form the force components of resultant forces in any direction in a plane parallel to the plane of the anode structure. The circuit is similar to FIGURE 1 except that the anode structure now consists of four quarters rather than two halves, as shown in the perspective view of FIGURE 7. For this operation, a ring discharge is employed, as represented in FIGURE 7 by ring 46. The anode is thus constructed of four anode portions 36, 37, 38 and 39 with resistors 40, 41, 42 and 43 respectively connecting anode portions 36, 37, 38 and 39 to the positive terminal of the energizing source 7'. Voltmeter 44 is connected between anode portions 36 and 37 such forces.

and voltmeter 45 is connected between anode portions 37 and 39. The remaining components are connected as in FIGURE 1 and are similarly designated with added prime notations. Voltmeter 44- is responsive to the same forces as voltmeter 11 of FIGURE 1. voltmeter 4-5 measures accelerating forces in a plane parallel to the plane of the anode structure and in a direction along the Y axis, as well as inclinations around the X axis. The operation of the circuit is similar to that of FIGURE 1, resulting in the measurement of accelerating forces in any direction in a plane parallel to the plane of the anode by treating separately the two right angle components of Similarly any resultant inclination around the X and Y axes may be measured.

The circuits of FIGURES l and 6 may be adapted as a velocimeter by coupling the difference voltages across the anode portions to an integrating circuit, the resulting integrated voltage being a measure of the velocity.

Although the embodiments of FIGURES 1 and 6 have been described primarily with relation to the employment of gas discharge devices, it should be recognized that a conducting liquid may be substituted for the gas, for example, a solution of water and salt. When considering applicants accelerometer circuits utilizing a liquid as the conducting medium within the discharge device;

an electrolytic conduction between the cathode and the anode portions replaces the former gaseous discharge. The energizing potential source may be appreciably lower than the gaseous embodiments since the liquid is considerably more conductive than the gas. A potential source of approximately 100 volts is typical. The remaining components may be of comparable value to those of the gaseous embodiment. The type of conducting path obtained in the liquid embodiment will normally be in the form of a r ng similar to the previously described ring discharge. It is preferred that the liquid completely fill the enclosed chamber of the discharge device so that turbulence of the liquid be minimized. The operation of the circuits using the liquid embodiment is essentially the same as previously described with the gas.

Although the invention has been described with relation to a number of specific embodiments to clearly demonstrate openability, it is not intended that the broad principles of the invention be in any way limited thereto;

Thus, applicants invention may also be useful as an angular accelerometer by utilizing the phenomenon of relative movement of the discharge path with respect to the anode portions in response to applied forces.

The appended claims are intended to be construed as including all modifications that come within the true scope of the invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. An electrical measuring device responsive to applied accelerational fields, comprising a cathode electrode and a plurality of anode electrodes surrounded by a thermally convective fluid medium having the property of being increasingly electrically conductive with increase in temperature, said electrodes being energized to cause current flow from the said anode electrodes to the said cathode electrode by conduction through the said fluid medium, the said anode electrodes being symmetrically positioned around an axis with respect to the said cathode so that convection cur-rents in the said medium will be syn.- metrical with respect to the said anode electrodes when the applied accelerational field is along the axis of symmetry of the said anode electrodes, and will be rendered unsymmetrical with respect to the said anode electrodes when a component of applied accelerational field is normal to the axis of symmetry of the said anode electrodes; and means coupled to the said anode electrodes for sensing the resulting differential conduction thereto so as to provide a measure of the said normal component of applied accelerational field.

2. An electrical measuring device responsive to applied acceleration forces comprising a cathode electrode having a restricted emissive area and a plurality of anode electrodes closely spaced and rigidly symmetrically positioned with respect to said cathode, said electrodes being enclosed in a sealed envelope filled with a thermally convective fluid medium having the property of becoming increasingly electrically conductive with increase in temperature and energized so as to cause a flow of electrical charges between said cathode and said anode electrodes, the flow to each of said anode electrodes being equal in the absence of said acceleration forces and unequal in response to said forces, and means coupled to said anode electrodes for sensing the resulting differential flow thereto so as to provide a measure of said applied acceleration forces.

3. A device as in claim 2 wherein said fluid is a gas.

4. A device as in claim 2 wherein said fluid is a conducting liquid.

5. An electrical measuring device responsive to applied acceleration forces in a predetermined direction comprising an elongated cathode having emission restricted to the tip thereof, a pair of closely spaced anodes in the shape of semicircular plates mounted in a plane orthogonal to the longitudinal axis of said cathode, the emissive tip of said cathode projecting through a central aperture formed by said closely spaced anode plates, said cathode and anode plates being enclosed in a hermetically sealed envelope filled with a thermally convective gaseous medium having the property of becoming increasingly electrically conductive with increase in temperature, a source of energizing potential coupled between each of said anode plates and said cathode to provide an ion flow between the emissive tip of said cathode and said anode plates, the flow to each of said anode plates being equal in the absence of said acceleration forces and unequal in response to said acceleration forces, and means coupled to said anode plates for sensing the resulting diiferential flow thereto so as to provide a measure of said applied acceleration forces.

6. An electrical measuring device responsive to acceleration forces applied in any direction in a predetermined plane comprising an elongated cathode having emission restricted to the tip thereof, four closely spaced anode plates in the shape of quadrants mounted in a plane orthogonal to the longitudinal axis of said cathode, the emissive tip of said cathode projecting through a central aperture formed by said closely spaced anode plates, said cathode and anode plates being enclosed in a hermetically sealed envelope filled with a thermally convective gaseous medium having the property of becoming increasingly electrically conductive with increase in temperature, a source of energizing potential coupled between each of said anode plates and said cathode to provide an ion flow between the emissive tip of said cathode and said anode plates, the flow to each of said anode plates being equal in the absence of said acceleration forces and unequal in response to said acceleration forces, and means coupled across the anodes of two orthogonally related pairs of anode plates for sensing the differential flow to the anode plates of each of said two pairs so as to provide a measure of the right angle force components of said applied acceleration forces.

7. An electrical measuring device responsive to lacceleration forces comprising an enclosed envelope filled with a thermally convective fluid atmosphere having the property of becoming increasingly electrically conductive with increasing temperature and having supported therein a first electrode having a restricted emissive area and a plurality of closely spaced second electrodes symmetrically located around the said emissive area of the said first electrode as a center, a source of energizing potential being coupled by a plurality of load resistors 10 respectively to the said plurality of second electrodes, the References Cited in the file of this patent said energizing potential producing current flow between UNITED STATES PATENTS the said first electrode and the said plurality of second a o electrodes equally divided among said plurality of sec- Case 1919 0nd electrodes in the absence of said acceleration forces, 5 2457620 Abraham 1948 and unsymmetrical convection currents in the said at- 2,685,025 Root July 1954 mosphere resulting from application of said acceleration 27l8610 Krawmkfil Sept 1955 forces producing unequal division of current among the OTHER REFERENCES said plurality of second electrodes; and means connected A ti le Tub Electronique, Poml Mesure dog to the said plurality of second electrodes for measuring 10 Accelerations, from issue 168 of Mesures et Controle the said unequal division of current. lndustriel, May 1951, page 202. 

1. AN ELECTRICAL MEASURING DEVICE RESPONSIVE TO APPLIED ACCELERATIONAL FIELDS, COMPRISING A CATHODE ELECTRODE AND A PLURALITY OF ANODE ELECTRODES SURROUNDED BY A THERMALLY CONVECTIVE FLUID MEDIUM HAVING THE PROPERTY OF BEING INCREASINGLY ELECTRICALLY CONDUCTIVE WITH INCREASE IN TEMPERATURE, SAID ELECTRODES BEING ENERGIZED TO CAUSE CURRENT FLOW FROM THE SAID ANODE ELECTRODES TO THE SAID CATHODE ELECTRODE BY CONDUCTION THROUGH THE SAID FLUID MEDIUM, THE SAID ANODE ELECTRODES BEING SYMMETRICALLY POSITONED AROUND AN AXIS WITH RESPECT TO THE SAID CATHODE SO THAT CONVECTION CURRENTS IN THE SAID MEDIUM WILL BE SYMMETRICAL WITH RESPECT TO THE SAID ANODE ELECTRODES WHEN THE APPLIED ACCELERATIONAL FIELD IS ALONG THE AXIS OF SYMMETRY OF THE SAID ANODE ELECTRODES, AND WILL BE RENDERED UNSYMMETRICAL WITH RESPECT TO THE SAID ANODE ELECTRODES WHEN A COMPONENT OF APPLIED ACCELERATIONAL FIELD IS NORMAL TO THE AXIS OF SYMMETRY OF THE SAID ANODE ELECTRODES; AND MEANS COUPLED TO THE SAID ANODE ELECTRODES FOR SENSING THE RESULTING DIFFERENTIAL CONDUCTION THERETO SO AS TO PROVIDE A MEASURE OF THE SAID NORMAL COMPONENT OF APPLIED ACCELERATIONAL FIELD. 